Formulating with Zinc Oxide

Zinc Oxide — An effective and versatile broad spectrum UV absorber for sunscreens and daily wear products

Zsolt Szabados and Patricia Aikens
BASF Corporation

Introduction

Zinc Oxide is an inorganic particulate UV filter long recognized for its safety and efficacy in broad spectrum protection in sunscreens. Awareness of solar — induced skin conditions such as cancer, immune system suppression, and pre-mature skin aging have pushed consumers to look for products containing UV filters to provide UVA screening as well as SPF. As a result we'll find UV filters not only in beach and sport products, but in daily wear preparations and color cosmetics as well. The growing market share of "green" and "natural" formulations, marketing campaigns, and consumer education stresses the need for formulators to look for innovative alternatives beyond the organic filters commonly used. Zinc Oxide provides broad spectrum protection and works synergistically with other UV filters. The innovation then lies in how we can effectively use Zinc Oxide to optimize the quality of sunscreen formulations we create with the many options available. Below we review the mode of action, characterization and benefits, as well as challenges and solutions for incorporating Zinc Oxide in effective aesthetically acceptable products for beach and daily wear products.
Fig. 1. Absorbance (A) vs. wavelength (λ) for a dispersion of Zinc Oxide (and) Triethoxycaprylyl silane
  Concentration in formulation: 7% Substrate: Plate, PMMA HD6/Mold
  Application amount: 0.75 mg/cm2 UVB/UVA Analyzer, Labsphere UV-2000S


Broad spectrum UV protection

It is widely recognized that broad spectrum protection from solar UVB and UVA irradiation (in the 320–400 nm range) is essential to help prevent skin cancer and pre-mature skin aging signs such as wrinkling and uneven pigmentation. Zinc Oxide is a highly effective broad spectrum UV filter providing coverage from UVB to far into the UVA I range. There are several organic UV filters that can reach into the UVA II — UVA I range, however Zinc Oxide is the only inorganic filter that can provide a critical wavelength (λmax) of 375nm, which is defined in the US FDA monograph as acceptable for a UVA protection claim. This is particularly important in more restricted markets, with limited choices of UVA filters, yet an increasing demand for broad spectrum products. Figure 1 shows the typical absorbance spectrum for a commercially available Zinc Oxide UV filter available in the marketplace.
As seen from Fig 1, a key advantage of Zinc Oxide is the extent of its UVA screening ability. It does however exhibit fairly low UVB absorption, and therefore the SPF is not very high. Zinc Oxide is compatible with a wide range of organic UV filters as well as titanium dioxide, so higher UVB protection can be achieved when used in various combinations to increase the SPF. For consumers who prefer an all-inorganic particulate sunscreen, the combination of zinc oxide and titanium dioxide is ideal for broad spectrum UV protection in both beach wear and daily wear products. In many cases there is synergy; for example, a combination of ethylhexyl triaxine, a strong UVB filter, with Zinc Oxide provides a much higher SPF than would be expected for either product alone, with UVA protection as well. Synergies such as these allow overall for lower levels of UV filters to be used, thus decreasing the cost, enhancing the efficacy, and improving the overall aesthetics.
Besides its broad spectrum coverage, Zinc Oxide has several other advantages over some organic UV filters(1). One is the exceptional photo-stability properties it exhibits; there is no decomposition or free radical generation with long exposure to sunlight with Zinc Oxide(2). Avobenzone and some combinations of it with other UV filters can photo-decompose without the addition of stabilizers(2). Also, in contrast to titanium dioxide, which requires surface treatment to be suitable for sunscreen use, Zinc Oxide can be used with or without a coating. Oxybenzone is an organic UV filter that extends to a low level into the near UV range, however it has the potential for skin irritation in sensitive individuals(3). Since Zinc Oxide is a particulate filter, it has very low potential for skin penetration beyond the stratum corneum(4,5). This makes it especially well-suited for mild sunscreens designed for child / baby skin.
Most common organic UV filters are oil-based substances and thus have the potential to leave a heavy, greasy, feel on the skin if not properly formulated. This would be expected to some extent in high SPF water-resistant beach and sport products, but is not acceptable aesthetically in daily wear moisturizers and foundations. The use of inorganic particulate filters such as Zinc Oxide for UVB and UVA protection gives a light dry skin feeling and allows for optimization of aesthetics based on chosen emollients. An additional practical benefit is that unlike some organic UV filters which can leave severe discoloration on clothing upon contact, Zinc Oxide does not cause staining.

Mechanism of UV screening for Zinc Oxide

The vast majority of UV filters based on organic chemistry function via absorption of high energy UV radiation though their conjugated chemical structure and conversion to benign infrared release. The mechanism of action for particulate inorganic filters is a bit more complex(6). Zinc Oxide absorbs UV radiation through a band gap mechanism, and it also scatters, reflects, and extends the path length of the incoming UV radiation within the applied film as seen in Fig. 2. Particulate type filters are not molecularly dissolved in the carrier and thus offer those two modes of screening. Some other UV filters such as titanium dioxide, bisoctrizole, and tris-biphenyl triazine also function in this way.
Figure 2 image
Fig. 2 Mode of action of particulate UV filters; In addition to absorption of UV radiation, particulate filters
can scatter, reflect, and extend the path length.


As shown in the illustration to the left, particulate filters are not dissolved in the medium, but remain in the film on the surface of the skin. This minimizes the potential for transfer beyond the stratum corneum. For this type of formulation, uniformity of the film for complete skin coverage is essential to maximize the UV screening performance.


Characterization of Zinc Oxide for use as a UV filter

In the USA, Zinc Oxide for use in sunscreens is classified as an active ingredient for over-the-counter drug use because the claim of prevention of sunburn applies, and therefore must comply with USP and the FDA sunscreen monograph. The INCI names include any surface coating treatment present in addition to Zinc Oxide.
Pigment grade Zinc Oxide is most often associated with enhancing opacity and skin coverage in color cosmetic make-up applications. Zinc Oxide used for UV protection, however is produced in a different way because the sunscreen needs to be transparent in appearance on the skin and not show any whitening or opacifying effect. This is essential for beach products and daily wear moisturizers. Traditionally it has been expected that the particle size must be small enough so that it would not reflect light in the visible range of 400 + nm which would give an undesirable whitening appearance. This would generally put the diameter in the nanoparticle range and there are several commercial Zinc Oxide products like this. There has been no credible scientific evidence published to confirm that there is any health hazard with the topical application of nano-particulate sized Zinc Oxide and in fact there are many studies concluding that no skin penetration beyond the stratum corneum occurs with topical application(4-6). In response to consumer concerns however about the potential for safety and health issues with nanoparticles and public perceptions, recent technologies have been developed to produce Zinc Oxide for sunscreens in the non-nanoparticle range, with no particles detected at less than100 nm in all dimensions.
The determination of the particle size of Zinc Oxide can be done in several ways. The most widely accepted is Dynamic Light Scattering method (DLS). An example is shown below in Fig. 3 for two grades of Zinc Oxide with the same surface treatment, (triethoxycaprylyl silane).
Figure 3 Image

Fig. 3 DLS for particle size analysis of 2 grades of Zinc Oxide (and) triethoxycaprylyl silane.
(Red = Z-COTE® HP1 with ave dia 120 nm. Green = Z-COTE® LSA with ave dia 420 nm)
Other forms of particle size analysis include Static Light Scattering (SLS) and BET, a gas adsorption technique, both of which are highly dependent on particle shape. These techniques have inherent inaccuracies when measuring Zinc Oxide because it tends to have an irregular shape and calculations are based on the assumption that the particle is spherical. BET in particular would calculate the particle size to be much lower depending on the extent of deviation a spherical shape. The ASTM approved method of DLS is considered the most widely accepted. Transition Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) can also be useful for visually assessing the average size of a small sample range.


Zinc Oxide is commercially available in two physical forms, powders and dispersions, so there are options to select from for development. Powders are traditionally known to be more time consuming to process, however they do offer some advantages and flexibility over the pre-processed dispersions, such as the choice to optimize the emollient combination for aesthetics and stability.

Formulating with Zinc Oxide as a UV filter

Zinc Oxide powders are available in different particle size ranges and with different surface treatments. The uncoated grade of Zinc Oxide is amphiphilic in nature, and it can be added in the continuous or dispersed phase of the emulsion versus the hydrophobically modified grades, which exclusively go into the oil phase of the preparation. Usually the coating varies between 1-4 %wt.
Besides adding Zinc Oxide in the designated phase, there are several other factors reviewed below which affect the stability, performance, and the cosmetic appeal of the formulation:

pH stability

It is very important to consider the pH of the system when using Zinc Oxide. At pH below 7, divalent zinc ions will migrate into the water phase and cause instability in the system, as Zn2+ can interact with various polymers (rheology modifiers and film formers) and emulsifiers (e.g.salts of stearic acid). Many acrylate-derived polymers are sensitive to Zn2+, forming complexes leading to the disruption of the polymer matrix and loss of stability. These reactions are even more pronounced with the uncoated Zinc Oxide. It has been found that chelating agents (e.g. Na2EDTA) and some salts (e.g. NaCl) in the aqueous phase can help reduce the extent of these interactions. During formulation, the pH of the system may rise, therefore it is recommended that buffering agents such as citric or lactic acid, or others be used.
Non-ionic emulsifiers and thickeners are the best choice when developing inorganic sunscreen formulations. The most commonly used are xantham gum, different grade of clays, hydroxyethyl cellulose, hydroxypropylmethyl cellulose and polyurethane-39.
Depending on the surface treatment of the Zinc Oxide, there can be exceptions when it comes to interactions with polyacrylate-based rheology modifiers. One of these is a grade of Zinc Oxide coated with the crosspolymer of triethoxycaprylylsilane and dimethoxydipheylsilane. The optimized hydrophobicity of this coating helps to prevent the migration of Zn2+ by allowing the particles to remain well dispersed within the oil droplet of the emulsion rather than migrating to the O/W interface and releasing ions into the water phase.

Emulsifiers

Zinc Oxide can be readily incorporated into water-in-oil (W/O) or oil-in-water (O/W) emulsion systems. Usually water-in-oil emulsions provide higher UV performance due to a more consistent film on the skin, however equally effective O/W products can be formulated, which provide a lighter more luxurious feel that is highly valued by the consumer. Caution should be exercised when working with ionic emulsifiers, especially in O/W systems as they might cause a shift in the pH (up or down) which can affect the stability of the formulation, impacting the cosmetic appeal and UV screening performance of the product. Non-ionic emulsifiers such as stearates, glucosides, oleates, etc. are then the obvious choice.

Density and loading

Generally the amount of Zinc Oxide incorporated is more of a concern with O/W emulsions. At very high loading of Zinc Oxide, the dispersed oil droplets can become denser than the water phase and therefore there is a need of increasing emulsifiers and/or rheology modifiers to prevent agglomeration and settling of the droplets.

Homogeneity and uniformity

The key to optimizing the UV performance and cosmetic appeal of the final product is to wet and disperse the Zinc Oxide particles, usually in the oil phase, prior to emulsification. This is a major step in the process, as it breaks apart large agglomerates which then may be stabilized by dispersants and/or the oil phase compatible with the coating. Although the use of polar emollients is not imperative with Zinc Oxide, they may affect the spreadability and can also aid in solubilizing organic UV filters that are prone to crystallization if they are part of the sunscreen combination used. The polarity of the emollients affects the dispersions of the uncoated grades of Zinc Oxide — the more polar the emollient the better the dispersion. The molecular weight and structure also have an impact. Usually nano-particles of Zinc Oxide require longer shear times (3–5 min) than larger particles (1–3 min), since there is a lot more surface area to wet.
Fig 4 below shows the difference between below between a poor dispersion (A) and a good one (B).
Figure 4A Image Figure 4B Image
Fig. 4 Optical microscopy of dispersion differences for Zinc Oxide in an emulsion
(A): Inadequate dispersion (B): Complete dispersion
As show in the photomicrographs above, formulators would want to achieve the uniformity illustrated in figure B. Some of these emollients include various caprylates (e.g. propylheptyl caprylate), carbonates (e.g. dicaprylyl carbonate), glycerides (e.g. cocoglycerides) or even low molecular weight hydrogenated polyisobutene.
Spreadability is characterized by many manufacturers as spreading speed, which is usually established by placing a controlled amount of emollient on a substrate and then measuring the distance travelled in unit time. Therefore we have high, medium and low spreading compounds. Usually higher spreading emollients are especially effective and help generating more uniform film on the skin, and thus better UV performance. High shear mixing (homogenization) of the emulsion is another key factor for stability and ensuring a uniform film upon application.

Transparency and cosmetic appeal

In addition to their excellent UV attenuation, nano-sized Zinc Oxide formulations give a transparent film on the skin, which comes very close in cosmetic appeal to the organic UV filters. Nano-particles, (defined as having at least one dimension below 100 nm) do not scatter visible light wavelengths, thus conferring transparency and elegance to the formulations when spread on the skin. When using high levels of Zinc Oxide however, several other formulation components can accentuate the “non-transparency” or “whitening”, such as the excess of emulsifiers, which can cause foaming during application. Even though xantham gum is one of the rheological polymers of choice when using Zinc Oxide, due to its secondary emulsifying properties it may increase foaming, causing an additive effect of whitening. De-foaming agents such as silicones or different esters (e.g. tridecyl neopentanoate) can be used to counter act the secondary whitening effect.
In addition, there are several grades of non-nano sized Zinc Oxide which can be readily incorporated into elegant and market trendy formulations. Figure 3 above shows particle size difference between nano and non-nano Zinc Oxide grades. With proprietary manufacturing processes, these larger sized Zinc Oxide grades are designed to be formulated in a similar way to the nano-sized grades and to give the same degree of transparency on the skin and UV protection. In both cases the higher the load of Zinc Oxide, the greater the potential of skin whitening, and the above mentioned "whitening" reducing aids can be employed.

Formulating with Zinc Oxide dispersions

All the above factors influence formulations containing Zinc Oxide in the commercially available pre-dispersed form as well. It is easier to incorporate these products into formulations since the wetting step is eliminated, however there are other variables which can influence the stability and performance of the preparation.
The formulator would not have flexibility in choosing their dispersing aids or emollients which are whatever the manufacturers found to be better for the stability of the dispersion and so the balance between the oil phase and water phase may need some adjustment. In order to counteract a greasy, draggy, or tacky feel of some standard dispersing aids, formulators need to may use extra emollients, silicones, and spherical powders, which will require a revision of the emulsifier and polymer level and emollient type. Another consideration when using Zinc Oxide dispersions is that some markets require a completely naturally derived ingredient list, which may be prohibitive when it comes to these compounds.

Conclusion

Because of the ease of use, its broad spectrum UV screening, and inherent photo-stability, Zinc Oxide is an excellent choice for sunscreen formulation, especially when combined many of the other UV filters in the marketplace. It offers flexibility in formulating which complements the total package Zinc Oxide provides — effective, gentle, and aesthetically appealing beach and daily wear formulations.

References

  1. M. A. Mitchnick , D. Fairhurst, SR.Pinnell; Microfine Zinc Oxide (Z-cote) as a photostable UVA/UVB sunblock agent. J Am Acad Dermatol. (1999) 40(1):85–90.
  2. D. Beasley and T. Meyer; Characterization of the UVA protection provided by avobenzone, Zinc Oxide, and titanium dioxide in broad-spectrum sunscreen products. Am J Clin Dermatol. (2010) 11(6):413–21
  3. EWG 2013 Guide to sunscreens www.ewg.org/2013sunscreen
  4. A.O. Gamer, E. Leibold, B. van Ravenzwaay; The in vitro absorption of microfine Zinc Oxide and titanium dioxide through porcine skin. Toxicology in Vitro (2006) 20: 301–307
  5. N.A. Monteiro-Riviere et al., Safety Evaluation of Sunscreen Formulations Containing Titanium Dioxide and Zinc Oxide Nanoparticles in UVB Sunburned Skin: An In Vitro and In Vivo Study. Toxicol. Sci. (2011) 6: 1–48
  6. D. Fairhurst and M.A. Mitchnick ; Chapter 17 — Particulate Sun Blocks; General Principles in Sunscreens: Development, Evaluation, and Regulatory Aspects, 2nd Ed. (Editors — N. Lowe, N. Shaath, M. Pathak) (1997): 313–352.
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